1. Field of the Invention
The present invention relates generally to a method for making glass, and particularly to a method for making high purity glass bodies.
2. Technical Background
Integrated circuits (ICs) are fabricated using microlithographic systems. The goal of IC manufacturers is to produce integrated circuits having linewidths as small as possible. Most of the microlithographic systems currently in use employ transmission optics having an illumination source coupled to an illumination optics assembly. These system components provide a photomask with illumination light. The illumination optics expand and collimate the laser light to thereby homogenize the light intensity. The photomask includes the image of an integrated circuit disposed thereon. The photomask is positioned between the illumination optics and a projection optical system. The projection optical system projects the image of the integrated circuit onto the semiconductor substrate to thereby expose the substrate. Both the illumination optical system, the photomask, and the projection optical system employ transmission optics. One way of reducing the linewidth in systems that employ transmission optics is to reduce the wavelength of the laser light source.
It was once thought the limit of making integrated circuits using transmission optics would be somewhere around one micron. However, with illumination light sources being capable of transmitting much shorter wave lengths, one-tenth micron feature sizes and smaller are currently being produced. For example, KrF lasers, which operate at a wavelength of 248 nm, are capable of producing integrated circuits having linewidths approaching 100 nm. ArF lasers represent an improvement over KrF lasers, operating at a wavelength of 193 nm. With improvements to the transmission optics, integrated circuits can theoretically be produced with linewidths near 70 nm. Designers are now considering F2 lasers. These lasers operate at a wavelength of 157 nm. F2 lasers hold the promise of producing integrated circuits having linewidths on the order of 50 nm.
While it may be possible to further reduce the operating wavelength of light sources used in illumination systems, the very use of transmission optics is becoming a limiting factor. The problem is that the glass materials currently employed are not transparent at shorter wavelengths. Integrated circuit manufacturers have seen this problem coming for some time and are investigating ways of overcoming the above described limitations.
In one very promising approach, designers are considering reflective optical microlithographic systems that employ extreme ultraviolet (EUV) illumination sources. EUV systems operate at wavelengths in an approximate range between 11 nm and 13 nm. Instead of transmitting light through lens systems, reflective optical systems employ mirrors to direct the light onto the semiconductor substrate. The photomasks used in EUV systems are also reflective. Because the wavelengths in EUV systems are so short, any irregularity present on the surface of a mirror will significantly degrade system performance. Thus, the optical blanks used to produce EUV mirrors must be of the highest quality.
Quality optical blanks useful in current microlithographic systems are being produced using a flame hydrolysis process. A mixture of very pure silica precursor and a titania precursor are delivered in vapor form to a flame. The precursors react to form SiO2xe2x80x94TiO2 soot particles. The soot particles melt in layers forming a solid fused silica optical blank. While this method can be used to produce high quality optical components for optically transmissive devices, this method has drawbacks when it is used to make EUV reflective optical components.
One problem being encountered in the fabrication of mirrors suitable for EUV applications is the presence of striae in the optical blank. FIG. 1 is a photograph showing an optical blank 1 made in accordance with the flame hydrolysis process described above. As shown, optical blank 1 includes striae 2 disposed therein. The striae 2 are formed as layers of soot are deposited during the flame hydrolysis process. Striae 2 are less of an issue in photomasks than in optical mirror components. Another problem associated with striae 2 is known as springback. Springback refers to the shape change of a glass object with a non-uniform coefficient of thermal expansion (CTE). The change in shape typically occurs upon removal of material from the glass object. Another problem relates to the presence and distribution of inclusions within the glass. Inclusions are either solid impurities or gaseous bubbles disposed within the glass. The occurrence of inclusions in glass made using the flame hydrolysis process is low, but improvements are always desired especially for EUV applications.
A method is needed to produce optical blanks having substantially no striae and no low frequency thermal expansion variations. A method is needed to produce low expansion glass suitable for various applications including transmission optics over a wide range of wavelengths. What is also needed is a method for producing EUV compatible optical blanks for use in reflective microlithography. A method is also needed that enables the production of large optical devices without cracking.
The present invention relates to a method for producing optical blanks for EUV microlithographic components. The present invention provides a method for producing homogeneous optical blanks having substantially no striae. The method of the present invention produces dense, substantially inclusion free glass. As a result, scattering is substantially reduced when EUV light is reflected from a component produced from the optical blank. Furthermore, the method of the present invention is also advantageous because it enables the production of large optical blanks that are not susceptible to cracking.
One aspect of the present invention is a method for forming a glass body. The method includes providing a glass aggregate. The glass aggregate is mixed with a liquid to form a slurry. The slurry is cast in a mold to form a porous pre-form. The mold includes a porous glass substrate. The porous pre-form is consolidated into a glass object.
In another embodiment, the present invention is a method for forming a glass body. The method includes providing a glass aggregate. The glass aggregate is mixed with a liquid to form a slurry. The slurry is pressure cast in a mold to form a porous pre-form. The porous pre-form is heated to form a glass object.
In another embodiment, the present invention is a method for forming a glass body. The method includes providing glass particles. The glass particles include relatively fine glass soot particles mixed with relatively coarse glass particles. The glass particles are mixed with a liquid to form a slurry. The slurry is pressure cast in a mold to form a porous pre-form. The porous pre-form is heated to form a glass object.
In another embodiment, the present invention is a method for forming a glass body. The method includes providing glass particles, the particles including relatively fine glass soot particles mixed with relatively coarse glass particles. The glass particles are mixed with a liquid to form a slurry. A mold is provided that has a glass substrate. The slurry is pressure cast in the mold to form a porous pre-form. The porous pre-form is consolidated to form a glass object.
In another embodiment, the present invention is a method for forming a glass body. The method includes providing a glass aggregate. The glass aggregate is mixed with a liquid to form a slurry. The slurry is pressure cast in a mold to form a porous pre-form. The porous pre-form is disposed in a chlorine gas atmosphere heated to a predetermined temperature. The chlorine gas reacts with the impurities for a pre-determined time, whereby the impurities are vaporized and carried out of the porous pre-form. The porous pre-form is consolidated to form a glass object.
In another embodiment, the present invention is a method for forming a glass body. The method includes providing glass particles, the particles including relatively fine glass soot particles mixed with relatively coarse glass particles. The glass particles are mixed with a liquid to form a slurry. The slurry is pressure cast in a mold to form a porous pre-form. The porous pre-form is disposed in a chlorine gas atmosphere heated to a predetermined temperature, the chlorine gas reacting with the impurities for a pre-determined time, whereby the impurities are vaporized and carried out of the porous pre-form. The porous pre-form is consolidated to form a glass object.
In another embodiment, the present invention is a method for forming a glass body. The method includes the step of providing a glass aggregate. The glass aggregate is mixed with a liquid to form a slurry. The slurry is cast in a mold to form a porous pre-form, the mold including a porous glass substrate having the same composition as the glass aggregate. The porous pre-form is consolidated into the glass body.
In another embodiment, the present invention is a method for forming a glass body. The method includes the step of forming soot particles as a by-product of a flame hydrolysis process. The soot particles are mixed with a liquid to form a slurry. Coarse glass cutlet is disposed in a mold. The slurry is vacuum cast in a mold to form a porous pre-form. The mold includes a porous glass substrate. The porous pre-form is consolidated into the glass body.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.